bootstrap: clippy: set TESTNAME based on given paths

This addresses #104200 by setting the TESTNAME environment
variable automatically based on the paths from run configs,
marking a selected set of UI tests to be run.

Note that this does not filter out other unit tests using #[test].

Author: xtexx

src/bootstrap/src/core/build_steps/test.rs

@@ -744,7 +744,7 @@ impl Step for Clippy {
     const DEFAULT: bool = false;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.path("src/tools/clippy")
+        run.suite_path("src/tools/clippy/tests").path("src/tools/clippy")
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -788,6 +788,26 @@ impl Step for Clippy {
         let host_libs = builder.stage_out(compiler, Mode::ToolRustc).join(builder.cargo_dir());
         cargo.env("HOST_LIBS", host_libs);
 
+        // Collect paths of tests to run
+        let paths = match &builder.config.cmd {
+            Subcommand::Test { .. } => &builder.config.paths[..],
+            _ => &[],
+        };
+        'partially_test: {
+            let mut test_names = Vec::new();
+            for path in paths {
+                if let Some(path) =
+                    helpers::is_valid_test_suite_arg(path, "src/tools/clippy/tests", builder)
+                {
+                    test_names.push(path);
+                } else if path.ends_with("src/tools/clippy") {
+                    // When src/tools/clippy is called directly, all tests should be run.
+                    break 'partially_test;
+                }
+            }
+            cargo.env("TESTNAME", test_names.join(","));
+        }
+
         cargo.add_rustc_lib_path(builder);
         let cargo = prepare_cargo_test(cargo, &[], &[], host, builder);